This question brings more questins. Are you ever going to hunt with the rifle? If so I would use what ever bullet you intend to hunt with so you can get drops worked out and get wind dope down.

What distances? The bottom line is the 250 will start out faster and flatter unless you are going long, then in the end the 300 will be the faster bullet with less drift.

I see things like this. I use my rifles to their full potential as far as distance. Also I am a hunter first then a target shooter. I know not all my hunting shots will be at max range, but if that shot presents it self I want all the help from a bullet I can get. That especially includes wind drift, retaned energy and retained velocity. This is why we have been shooting 230's in my sons 300 win. It starts slower than a 210 but by 1000 yards the 230 is now passing the 210 with retained velocity and is now carrying 130 lbs more energy.

I shoot long range alot and have for years. You won't find many of the lighter for caliber offerings in my bullet cabinet. If you intend to use that 338 LM for anywhere near the distance it is accurate to, I would not sell it short with a lighter bullet. I got over the " faster flatter" thing long ago.

Thats just my opnion from what I see in the field with side by side testing. However you will find forum experts with a different opinion I am sure.